1. Field of the Invention
This invention relates to weapon-locating systems that track the path of incoming airborne targets back to the source of fire and position of the weapon, and more specifically to a new class of weapon-locating lidar (LIght Detection And Ranging) aka ladar system that uses flow field measurements to backtrack the wake turbulence trailing the airborne target from the point the target is detected backwards to estimate a backward trajectory. The backward trajectory can be used to estimate the point-of-origin (POO) of the target. The flow field measurements may also be used to classify the airborne target, which can be used to refine the POO estimate or to influence the counter-fire directed at the POO.
2. Description of the Related Art
Weapon-locating radar (RAdio Detection And Ranging) systems track the path of incoming projectiles including shells, rockets, mortars, missiles etc., and calculate the point from which the projectile was fired. These weapon-locating systems use Doppler radar to detect the hardbody of the projectile and then track the position of the hardbody forwards over a latter portion of the projectile trajectory. These systems typically assume a ballistic trajectory to backtrack along the estimated flightpath to the Earth intercept to estimate the point-of-origin (POO) for effective counter-fire tactics. The weapon-locating system also predicts impact zones and transmits data to friendly forces, allowing time for effective defense measures. Sized for easy transport, weapon-locating systems are valued for their accuracy, mobility, reliability and low life-cycle costs. Weapon-locating radar systems are currently available in two general classes of sensors, intermediate and long range. Examples include Raytheon's TPQ-36 Firefinder system is specifically designed to counter medium range enemy weapon systems out to a range of 24 kilometers, while the TPQ-37 Firefinder system can locate longer-range systems, and even surface launched missiles, out to 50 kilometers.
Referring now to FIGS. 1a-1c, in a typical battlefield scenario an enemy artillery piece 10 hidden behind a hill, dune or treeline 12 fires a projectile 14 at friendly forces. To direct effective counter-fire at the artillery piece 10, the location of the artillery piece as the POO of the projectile fire must be determined. A weapon-locating Doppler radar system 16 such as the TPQ-36 or TPQ-37 scans a microwave energy beam 18 that covers a narrow instantaneous field-of-view (FOV) over a large field-of-regard (FOR) 22 to detect and then track incoming projectiles 14. A Doppler radar system analyzes how the frequency of the returned signal has been altered by the motion of the projectile. This variation gives direct and accurate measurements of the projectile's radial velocity relative to the radar system. Doppler radar can provide 3D position (e.g. coarse measurements of the Azimuth angle and Elevation angle and precise estimates range) of the target hardbody.
Once a stable track is established, the weapon-locating radar system follows the forward trajectory of the hardbody projectile 14 to measure a number of track points 24. The weapon-locating system assumes a ballistic trajectory and applies ballistic calculations to the track points 24 to backtrack along an estimated ballistic trajectory 25 to the Earth intercept to estimate a POO 26. The POO 26 is passed to a counterbattery 28 that computes a firing solution and directs counter-fire (e.g. a projectile 30) at the artillery piece 10 at the estimated POO.
Ideally the weapon-locating system would detect and track the projectile from the point the projectile is first observable by the radar system, i.e., the point the projectile emerges from behind the hill, dune or treeline 12 or if there is a clear line-of-sight, the point the projectile is fired from the artillery piece 10. In practice there is a delay, corresponding to many meters of distance travelled, before the system can detect the projectile and establish a stable track. Factors that contribute to this delay include a finite amount of time that it takes for the narrow FOV 20 of scanned beam 18 to cross the flight path of the projectile 14 and detect the hardbody of the projectile. The radar cross section of certain projectiles may be low enough that the projectile is not detected on the first pass. An additional scan or two may be required after initial detection to establish the stable track.
Doppler radar can provide a reasonably good estimate of the POO if the projectile is detected and tracked early enough in its flight trajectory, if the projectile does not boost or maneuver during any portion of the flight path (i.e. it does not depart from a pure ballistic trajectory) and if its flight path is not disturbed by high winds or unstable air. If these conditions are not met, the estimated POO will be less accurate, and the counterbattery fire will be less effective.